METHODS AND SYSTEMS FOR REDUCED NOx COMBUSTION OF COAL WITH INJECTION OF HEATED NITROGEN-CONTAINING GAS

ABSTRACT

A stream of nitrogen-containing gas is heated and injected into a stream of coal and conveying gas to produce a stream of mixed nitrogen-containing gas, coal, and conveying gas. Oxygen is injected into the stream of mixed nitrogen-containing gas, coal, and conveying gas to produce a stream of mixed nitrogen-containing gas, coal, conveying gas, and oxygen. The mixed nitrogen-containing gas, coal, conveying gas, and oxygen are combusted in a combustion chamber.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. § 119(e) toprovisional application No. 60/742,119, filed Dec. 2, 2005, the entirecontents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

NOx generally refers to nitrogen monoxide NO and nitrogen dioxide NO2.Both are major contributors to acid rain and smog (ground level ozone)issues. The NOx partition in the flue gases of pulverized coal boilersis typically more than 95% NO and the remainder NO2 (Mitchell S. C., NOxin Pulverized Coal Combustion, IEA Clean Coal Center Report CCC/05,1998). During coal-combustion, the NOx production originates from threedifferent mechanisms:

Fuel-NOx mechanism,

Thermal-NOx mechanism, and

Prompt-NOx mechanism.

In pulverized coal boilers, 70% to 80% of NOx is formed from thefuel-bound nitrogen species (ftiel-N) via the fuel-NOx mechanism, andthe remaining NOx is formed from atmospheric nitrogen (N2), via thethermal-NOx mechanism (5-25%) and via the prompt-NOx mechanism (lessthan 5%) (Wu Z., NOx controlfor pulverized coal-fired power stations,IEA Clean Coal Center Report CCC/69, 2002). Understanding and limitingthe NOx formation in pulverized coal combustion is therefore stronglyrelated to the fuel-N conversion mechanism. A complex series ofreactions explains the transformation of coal bound fuel-nitrogen intoNOx or N2, including more than 50 intermediate species and hundreds ofreactions.

The two main parameters affecting the fuel-NOx formation process are thevolatile matter content of the fuel and the stoichiometry (air/fuelratio). Coal nitrogen content (bound nitrogen only), also stronglyimpacts NOx emission levels. Coal typically contains 0.5% to 3% nitrogenby weight on a dry basis. For comparison, natural gas also contains somenitrogen (0.5 to 20%); however it is molecular nitrogen N2, and thus isnot affected by the fuel-NOx mechanism.

FIG. 1 summarizes the main reactions affecting fuel-nitrogen in thecombustion process (Zevenhoven R., Kilpinen P., Control of pollutants influe gases and fuel gases, Picaset Oy, Espoo, ISBN 951-22-5527-8, 2001).Four main steps can be identified:

1—Devolatilization releasing coal nitrogen compounds (coal-N) in agaseous phase (Volatile-N), mainly as HCN, some as NHi. The remainingcoal-nitrogen compounds stay in the solid phase (char), and are referredto as char-N,

2—HCN evolution to NHi species,

3—NHi oxidation to NO or reduction to N2 depending on local conditions,and

4—Reburning, as some NO is recirculated back to the hot reducing zone ofthe flame and converted back to N2 while contacting CHi radicals.

Both volatile-N and char-N can be evolved as NO or as N2. Fuel-NOxformation is minimized by implementing specific conditions leading to N2rather than NO (see Van Der Lans R. P., Glarborg P. and Dam-Johansen K.,Influence of process parameters on nitrogen oxide formation inpulverized coal burners, Prog. Energy Combust. Sci. Vol. 23, p. 349-377,1997; Bowman C. T., Kinetics of Pollutant Formation and Destruction onCombustion, Prog Energy Combust Sci 1 33-45, 1975; and Proceedings ofthe 6th International Conference on Technologies and Combustion for aCleaner Environment, Oporto, Portugal, 2001).

For a given coal and particle size, three main conditions willindependently or in combination promote fuel-bound nitrogen conversioninto molecular nitrogen N2 rather than NO:

Fuel rich (reducing) conditions at the burner level: by arrangingfuel-rich “zones” in the furnace during the devolatilization stage, thenitrogen species in gas phase (volatiles) are more likely to be reducedto molecular nitrogen (N2) rather than oxidized to NO.

High temperature in the early stages of combustion increases thevolatiles yield. As volatiles burn close to the burner exit, controllingthe volatile-N (gas) to N2 conversion is much easier than the char-N(solid) to N2 conversion. High temperature at the burner exit alsoincreases both the reburning rate of recirculated NO and the conversionrate of volatile-N into N2 (see Sarofim A. F., Pohl J. H., Taylor B. R.,Strategies for Controlling Nitrogen Oxide Emissions during Combustion ofNitrogen-bearing fuels, 69th Annual Meeting of the AIChe, Chicago, Ill.,1976; and Bose A. C., Dannecker K. M. and Wendt J. O. L., Energ. Fuel,Vol. 2, p. 301, 1988).

Long residence times in the high temperature and reducing zones in theboiler lead to higher fuel-N to N2 and NO to N2 conversion.

Prior research indicates that oxygen was used alone to decrease the NOxformation. Due to safety and other concerns, oxygen was injected at arelatively low temperature and also in the burner just before thecombustion.

BRIEF SUMMARY OF THE INVENTION

A method is provided and a system for performing the method. A stream ofnitrogen-containing gas is heated and injected into a stream of coal andconveying gas to produce a stream of mixed nitrogen-containing gas,coal, and conveying gas. The mixed nitrogen-containing gas, coal, andconveying gas are combusted with oxygen in a combustion chamber.

BRIEF DESCRIPTION OF THE DRAWINGS

For a further understanding of the nature and objects of the presentsystem and method, reference should be made to the following detaileddescription, taken in conjunction with the accompanying drawings, inwhich like elements are given the same or analogous reference numbersand wherein:

FIG. 1 is a schematic summarizing the main reactions affectingfuel-nitrogen in the combustion process; 100221 FIG. 2 is a schematicview of the system with oxygen injection upstream of the burner;

FIG. 3 is a schematic view of the system with oxygen injection at theburner;

FIG. 4 is a perspective view of a tubular injection element havingrectangular apertures;

FIG. 5A is a schematic of a circular aperture for use in a tubularinjection element;

FIG. 5B is a schematic of a rectangular aperture for use in a tubularinjection element;

FIG. 5C is a schematic of a triangular aperture for use in a tubularinjection element;

FIG. 5D is a schematic of an elliptical aperture for use in a tubularinjection element;

FIG. 6 is a perspective view of a tubular injection element having threesets of rectangular apertures;

FIG. 7 is a perspective view of a tubular injection element having threesets of decreasingly shorter rectangular apertures;

FIG. 8 is a perspective view of a tubular injection element havingrectangular apertures arranged in a staggered pattern;

FIG. 9 is a perspective view of a tubular injection element having avertically non-uniform distribution of rectangular apertures;

FIG. 10 is a perspective view of a tubular injection element having anaerodynamic pointed tip with rectangular apertures;

FIG. 11 is a perspective view of a tubular injection element having anaerodynamic rounded tip with rectangular apertures;

FIG. 12 is a perspective view of a tubular injection element having anaerodynamic rounded tip with elliptical apertures;

FIG. 13 is a perspective view of a tubular injection element having anaerodynamic pointed tip with elliptical apertures;

FIG. 14 is a cross-sectional view of two concentric injections withswirler-type injection elements;

FIG. 15A is a perspective view of two injections with a swirler disposedon the nitrogen lance and a tangentially injecting injection elementdisposed on an inner wall of the fuel duct wherein the swirl andtangential injections are generally in the same direction;

FIG. 15B is a perspective view of two injections with a swirler disposedon the nitrogen lance and a tangentially injecting injection elementdisposed on an inner wall of the fuel duct wherein the swirl andtangential injections are generally in the opposite direction;

FIG. 16 is a side elevation view of a swirler showing opening and wallwidths;

FIG. 17 is a perspective view (with the aerodynamic tip not illustrated)of four injection elements radially spaced from one another having a legwith at least one aperture at an end thereof;

FIG. 18 is a side elevation view (with the aerodynamic tip illustrated)of the injection element configuration of FIG. 17;

FIG. 19 is a front elevation view (with the aerodynamic tip illustrated)of the injection element configuration of FIG. 17;

FIG. 20 is a front elevation view of a two-injection elementconfiguration having a fin configuration;

FIG. 21 is a side elevation view of the two-injection elementconfiguration of FIG. 20;

FIG. 22A is a side elevation view of an axial injection element with avertically oriented, elliptical cross-sectional shape;

FIG. 22B is a side elevation view of an axial injection element with ahorizontally oriented, elliptical cross-sectional shape;

FIG. 23 is a perspective view of a tubular injection element havingthree radially spaced apertures at an end, thereof for injecting oxygenat an angle to the axis;

FIG. 24A is a side elevation view of a tubular injection element withapertures configured as circles arranged in a circle with one aperturein the middle;

FIG. 24B is a side elevation view of a tubular injection element with asaw tooth-shape pattern of apertures at a peripheral portion thereof;

FIG. 24C is a side elevation view of a tubular injection element with afour-wedge type pattern of apertures;

FIG. 24D is a side elevation view of a tubular injection element with astar-shaped aperture;

FIG. 24E is a side elevation view of a tubular injection element with acurved, cross-shaped aperture disposed at a center thereof; and

FIG. 24F is a side elevation view of a tubular injection element with acurved, cross-shaped aperture similar to that of FIG. 24E but having agreater thickness and extending to a peripheral portion thereof.

DETAILED DESCRIPTION

Increasing the temperature of the coal to safely release volatiles in anoxygen deficit environment prior to combustion can hinder the NOxformation mechanism. This is achieved in the proposed method and systemby injecting high temperature nitrogen-containing gas upstream of theburner and optionally by injecting oxygen-containing gas in the burneror just upstream of the burner. Injection of hot nitrogen-containing gasin an O2-deficit environment causes devolatilization of the coal torelease volatiles and can also decompose them to N2. The fuel-boundnitrogen in char can also be decomposed to N2. This process alsoincreases the residence time of the volatiles-N and char-N in the maincombustion zone thereby favoring decomposition to N2. Further NOxreductions are obtained by the optional injection of oxygen-containinggas in the burner or just upstream of the burner to compensate theinjected nitrogen and to increase the temperature of the flame in fuelrich condition. Both the processes promote formation of N2 instead ofNOx.

The system for burning coal with reduced NOx emissions includes thefollowing: a source of a mixture of coal and conveying gas; a source ofoxygen-containing gas; a source of nitrogen-containing gas; a heatingdevice adapted and configured to heat nitrogen from the nitrogen source;a combustion chamber; a burner disposed at a wall of the combustionchamber; a burner operatively associated with a combustion chamber; afuel duct in fluid communication with the source of a mixture of coaland conveying gas, the fuel duct extending towards the burner; and anitrogen-containing gas injection element in fluid communication withthe heating device and the fuel duct, the nitrogen injection elementbeing adapted and configured to inject heated nitrogen-containing gasfrom the heating device into a stream of a mixture of coal and conveyinggas and mix therewith inside the fuel duct.

A method of combusting coal with reduced NOx emissions includes thefollowing steps. A stream of nitrogen-containing gas is heated. Theheated stream of nitrogen-containing gas is injected into a stream ofcoal and conveying gas to produce a stream of mixed nitrogen-containinggas, coal, and conveying gas. The mixed nitrogen-containing gas, coal,and conveying gas are introduced at a burner disposed at a wall of acombustion chamber. The coal is combusted with oxygen in the combustionchamber.

The system or method can include any one or more of the followingaspects:

an oxygen-containing gas injection element is in fluid communicationwith the source of oxygen and the fuel duct, the oxygen-containing gasinjection element fluidly communicating with the fuel duct downstream ofwhere the nitrogen-containing gas injection element fluidly communicateswith the fuel duct and upstream of or at the burner, theoxygen-containing gas injection element being adapted and configured toinject oxygen-containing gas from the oxygen-containing gas source intothe stream of mixed heated nitrogen-containing gas, coal, and conveyinggas;

the source of oxygen-containing gas and the source ofnitrogen-containing gas comprise an Air Separation Unit (ASU);

the conveying gas comprises flue gas from the combustion chamber mixedwith oxygen-containing gas from the oxygen-containing gas source;

the heating device is adapted and configured to directly impart heat tonitrogen from the nitrogen-containing gas source from a flame;

the heating device is a heat exchanger adapted and configured toexchange heat between nitrogen-containing gas from thenitrogen-containing gas source and heat from combustion of the coal andoxygen in the combustion chamber;

the conveying gas is flue gas from the combustion chamber mixed withoxygen;

the heating device is a heat exchanger adapted and configured toexchange heat between nitrogen-containing gas from thenitrogen-containing gas source and heat from combustion of the coal andoxygen in the combustion chamber;

the heating device is a heat exchanger adapted and configured toexchange heat between nitrogen-containing gas from thenitrogen-containing gas source and heat from combustion of the coal andoxygen in the combustion chamber;

the conveying gas is air;

the oxygen-containing gas injection element fluidly communicates withthe fuel duct at the burner;

the nitrogen-containing gas injection element fluidly communicates withthe fuel duct at a peripheral portion of the fuel duct;

the nitrogen-containing gas injection element fluidly communicates withthe fuel duct along a central axis of the fuel duct;

the oxygen-containing gas injection element fluidly communicates withthe fuel duct at a peripheral portion of the fuel duct;

the oxygen-containing gas injection element fluidly communicates withthe fuel duct along a central axis of the fuel duct;

the nitrogen-containing gas and oxygen are obtained from an ASU;

the step of heating a nitrogen-containing gas stream comprises directlyimparting heat from a flame to the nitrogen stream;

the step of heating a nitrogen-,containing gas stream comprisesindirectly imparting heat from a flame to the nitrogen stream via a heatexchanger;

the step of heating a nitrogen-containing gas stream comprisesindirectly imparting heat from the step of combusting to the nitrogenstream via a heat exchanger;

the stream of nitrogen-containing gas is heated to a temperature suchthat a desired level of devolatilization occurs;

the stream of nitrogen-containing gas is heated to a temperature in therange of from about 1,000° F. to about 1,800° F.;

injection of the heated stream of nitrogen-containing gas causesdevolatilization of most of a volatile species content in the coal;

collecting at least some of any flue gas produced from the step ofcombusting, injecting oxygen-containing gas into the collected flue gasto mix therewith, and introducing the mixed oxygen and flue gas to theburner;

oxygen-containing gas is injected into the collected flue gas in anamount such that an oxygen concentration in the mixed oxygen-containinggas and flue gas is from about 3% to about 20%.; and

the step of heating a nitrogen-containing gas stream comprisesindirectly imparting heat from the step of combusting to the nitrogenstream via a heat exchanger.

The proposed method and system also reduces the fuel-NOx formation in acoal combustion process. As described in the above section, the fuelbound N can be transformed into either molecular N (N2) or NO dependingon the local conditions where the devolatilization took place. Injectinghot nitrogen-containing gas into the coal stream releases volatiles andfuel-bound N compounds in a reducing environment. The reducingenvironment drives the coal derived N compounds to convert to N2.

The temperature and quantity of nitrogen-containing gas to be injecteddepends on the type of coal and the NOx reduction targets. Thetemperature of the nitrogen-containing gas is chosen to be above thedevolatilization temperature of the volatile species in the coal. Thevolatilization characteristics of various general types of coals arewell known. In the case of a specific type of coal, the volatilizationcharacteristics may be determined experimentally in a known manner.Generally speaking, the temperature should be selected such that adesired degree of devolatilization occurs for the particular type ofcoal being combusted. A suitable temperature is in the range of fromabout 1,000° F. to about 1,800° F. The location of nitrogen-containinggas injection should be strategically placed so that just enoughresidence time is available for the devolatilization and the conversionto nitrogen-containing gas to occur. Injecting hot nitrogen-containinggas more than this distance can pose safety issues as volatiles are veryflammable and unfavorable combustion could occur.

The nitrogen-containing gas need not be pure nitrogen. Indeed, gaseousmixtures having a majority of nitrogen with minor amounts of other gasesare suitable for use with the process and system. Such minorconstituents include O2 and inert gases such as Ar and CO2. A preferredsource for both the nitrogen-containing gas to be heated and the O2 isfrom an air separation unit (ASU). Suitable ASU's include those operatedvia pressure swing adsorption (PSA), vacuum swing adsorption (VSA),cryogenic distillation, and membrane permeation. Typical N2 and O2concentrations in nitrogen-enriched and oxygen-enriched streams fromthese types of ASU's are well known and need not be repeated here. Othersources of the nitrogen can include a gaseous mixture comprisingnitrogen and flue gas.

The oxygen-containing gas to be optionally injected into the mixednitrogen-containing gas, conveying gas, and coal also need not be pure.Suitable gases include those having an oxygen concentration greater thanthat of air up to 100% pure oxygen.

In the case of N2 from an ASU, the nitrogen-containing gas can be heatedin “direct fired mode” or “indirect fired mode”. In a direct fired modethe incoming nitrogen-containing gas is heated by direct contact with asmall flame. In indirect fired mode, the nitrogen-containing gas isheated at a heat exchanger taking heat from a small flame or from acombustion process.

In fuel-rich flame conditions, injection of oxygen in the maincombustion zone increases the temperature. This higher temperaturereducing environment promotes formation of N2 from the remainingvolatiles released from the coal. If optional oxygen-containing gasinjection is selected, the oxygen-containing gas is injected at alocation which achieves both safety goals and good mixing with thestream of coal/conveying gas/nitrogen-containing gas. The location isdesirably upstream of the burner throat in order to reduce the risk ofincurring partial combustion of coal particles in local pockets that areoxygen-enriched. At the same time, the location is not so close to theburner that little mixing of the oxygen-containing gas andcoal/conveying gas/nitrogen-containing gas is achieved.

The conveying gas comprises any gas to convey fuel particles from aparticle storage or generation location, e.g., mills, to the burnerlevel and the combustion chamber. For example, this gas can comprise theprimary air used to convey pulverized or micronized coal in a coal-firedboiler. Preferred conveying gases are air and mixtures of recirculatedflue gas and oxygen. Typically, these mixtures of recirculated flue gasand oxygen include about 60-90% CO2, 5-20% N2, and 3-20% O2. Anespecially preferred mixture of recirculated flue gas and oxygencontains about 80% CO2 and about 20% O2.

Several different types of injection elements may be employed. It shouldbe noted that each of the nitrogen-containing gas and oxygen-containinggas injection elements may be the same as one another or different.Several examples of injection elements follow.

A system for performing the method is best illustrated in FIGS. 2-9. Asbest illustrated in FIG. 2, in one embodiment a stream of coal andconveying gas 1 enters fuel duct 8. A heated stream ofnitrogen-containing gas from first injection element 3 is mixed with thecoal and conveying gas downstream of element 3. Oxygen-containing gas isoptionally injected into the mixed coal, conveying gas, andnitrogen-containing gas by injection element 4 upstream of burner 9. Themixed nitrogen-containing gas, oxygen-containing gas (if optionallyinjected), coal, and conveying gas is introduced to combustion chamber 6via a burner where combustion 7 takes place.

As best illustrated in FIG. 3, in another embodiment a stream of coaland conveying gas 1 enters fuel duct 8. A heated stream ofnitrogen-containing gas from first injection element 3 is mixed with thecoal and conveying gas downstream of element 3. Oxygen-containing gasesoptionally injected into the mixed coal, conveying gas, andnitrogen-containing gas by injection element 5 at burner 9. The mixednitrogen-containing gas, oxygen-containing gas (if optionally injected),coal, and conveying gas is introduced to combustion chamber 6 via burner9 where combustion 7 takes place.

It should be noted that injection elements 3, 5 need not be disposedcentrally along an axis of the fuel duct 8. Rather, they may be disposedalong a peripheral portion of the fuel duct 8. Some of these variousconfigurations are best illustrated in some of the following injectionelement designs.

Radially Injecting Injection Elements Designs:

As illustrated in FIG. 4, one injection element 10 is a tube having aclosed end 16 and plurality of rectangular apertures 13. This designprovides radial injection from the circumferential face of the injectionelement 10.

The length, D1, and width, D2, of these apertures, as well as thecircumferential arc distance, D0, between two adjacent apertures may bevaried to control the momentum ratio J (ratio of the oxygen-containinggas or nitrogen-containing gas jet momentum to the momentum of thestream of non-gaseous fuel/conveying gas). D1, D2, and D0 also controlthe penetration of the injection gas into the primary stream or primarystream mixed with nitrogen-containing gas as appropriate. A small D2/D1ratio (streamlined rectangular apertures) will minimize the perturbationto solid fuel particles, such as coal. A big D2/D1 ratio (bluff-bodyslots) will have a greater influence on the solid phase and will pushsolid fuel particles, such as pulverized coal, away from the centerlineof the burner primary air duct. Those two different aspect ratios willlead to different distribution of particles and nitrogen or oxygen atthe duct outlet.

Those three parameters, S1, D1, and D2, in turn, control the penetrationof the injection gas into the primary stream or primary stream mixedwith nitrogen-containing gas as appropriate. A small D2/D1 ratio(streamlined slots) will minimize the perturbation to the solid phase. Abig D2/D1 ratio (bluff-body slots) will have a greater influence on thesolid phase and will push the coal particles away from the centerline ofthe burner primary air duct. Those two different aspect ratios will leadto different distribution of particles and nitrogen or oxygen at theduct outlet. As shown in FIGS. 5A-5D, the slot shape itself could becircular, rectangular, triangular, or elliptical, respectively.

As depicted in FIG. 6, the injection element 20 includes apertures 23arranged in axially extending rows along the axis of the injectionelement 20. This pattern performs a better mixing if the axial distanceD3 between two adjacent apertures 23 in a same row is sufficientlylarge. The dimension D3 between the apertures 23 could be the same orcould vary in the axial direction towards the closed end 26.

As best illustrated in FIG. 7, the length dimensions D1, D4, and D5 ofthe apertures 33 in injection element 30 may vary from short to longgoing in the direction of the closed end 36. Alternatively, these lengthdimensions could vary in any order from short to long, long to short,long to short and then back to long, short to long and then back toshort, and other permutations. In addition, the dimensions D1 or D2could also vary in the azimuthal (radial) direction. This offers moreprecise control over the penetration of the injection gas into theprimary stream. Finally, D3 can be tailored to the conditions of eachprocess to optimize mixing and minimal redistributions of particles.

As shown in FIG. 8, the apertures 43 in injection element 40 need notextend in the axial direction. Rather, they may be staggeredly disposedat different angles Θ with respect to one another. Θ can vary from lessthan 180° (streamlined slots/axial slots) to 90° (bluff-bodyslots/radial slots).

As depicted in FIG. 9, the injection element 50 need not have a uniformdistribution of apertures 53 in the azimuthal direction. As discussedpreviously, in coal-fired boilers, the coal particle loading is notalways uniform throughout the cross-section (sometimes due to theso-called “roping phenomenon”). In the case of coal, the particleconcentration in the stream of coal/conveying gas 56 (or coal/conveyinggas/nitrogen-containing gas) at the bottom of the injection element 50may be higher than the same in the stream of coal/conveying gas (orcoal/conveying gas/nitrogen-containing gas) 57 at the top of theinjection element 50. In this figure, the thickness of arrows representsthe loading of particles in the gas stream. The advantage offered bythis is that more nitrogen-containing gas or oxygen-containing gas couldbe introduced in the locations where particle loading is higher 58 thanlocations where particle loading is lower 59. This will reduce thelikelihood of creating local pockets with less devolatilizationpotential (in the case of nitrogen-containing gas injection) or localpockets that are fuel-lean (in the case of oxygen-containing gasinjection) each of which could lead to higher levels of NOx. Withrespect to this problem and solution, the particle loading distributioncould easily be determined by experimental or modeling studies.

Similar to the injection element designs 10, 20, 30, 40, the apertures53 may be staggered and vary in size in the axial and azimuthaldirections. The distance between apertures 53, the number of rows ofapertures 53, or the surface area of apertures 53 could also be varied.

This injection element 50 has a particularly beneficial application tocoal-fired boilers whose burner geometry include coal concentrators orsplitters (identified technique in the prior art for reducing NOxemissions from pulverized coal burners). Varying levels ofnitrogen-containing gas or oxygen-containing gas injection may belocated to achieve higher concentration of N2 or O2 in coal richerzones. As a result, the equivalence ratio between coal and N2 (in thecase of nitrogen-containing gas injection) coal and O2 (in the case ofoxygen-containing gas injection) can be controlled in the coal richerzone (concentrated zone) as well as in the coal leaner zones.

Aerodynamic Injection Element Designs:

As depicted in FIGS. 10-14, the injection element 100, 110, 120, 130,140 may have an aerodynamic closed end 106, 116, 126, 136, 146. Anaerodynamic shape tends to reduce re-circulation of the stream ofcoal/conveying gas (in the case of nitrogen-containing gas injection) orof the stream of coal/conveying gas/nitrogen-containing gas (in the caseof oxygen-containing gas injection), and creation of a particledeficient and low/reverse velocity zone in the wake of the injectionelement 100, 110, 120, 130, 140.

Referring to the injection element 100 of FIG. 10, rectangular apertures103 could be added to closed end 106 in all the permutations describedin FIGS. 1-7. The closed end 106 could be pointed, and terminate atpoint P1. The distances D8 and D9 and the angle α defined by lines L1and L2 could be varied in order to optimize the mixing in a shortestdistance and to cause least disturbance to the non-gaseous fuel.

Referring to the injection element 110 of FIG. 11, rectangular apertures113 could be added to closed end 116 in all the permutations describedin FIGS. 1-7. The closed end 116 could be rounded, instead of extendingto point P2 at the intersection of lines L4 and L5. The distances D10and D11, and the angle 6 defined by lines L4 and L6 could be varied inorder to optimize the mixing in a shortest distance and to cause leastdisturbance to the non-gaseous fuel.

As illustrated in FIG. 12, elliptical (or circular) apertures 123A,123B, 123C may be present on injection element 120. The injectionelement 120 extends to a rounded tip 126. Each of apertures 123A, 123B,and 123C is configured to inject streams of nitrogen-containing gas oroxygen-containing gas PA, PB, PC into the mixed stream of coal/conveyinggas (in the case of nitrogen-containing gas injection) or coal/conveyinggas/nitrogen-containing gas (in the case of oxygen-containing gasinjection) at an angle to the axis of the lance.

As shown in FIG. 13, elliptical (or circular) apertures 133A, 133B, 133Cmay be present on injection element 130. The injection element 130extends to a pointed tip 136. Each of apertures 133A, 133B, and 133C isconfigured to inject a stream of nitrogen-containing gas oroxygen-containing gas PD, PE, PF into the mixed stream of coal/conveyinggas (in the case of nitrogen-containing gas injection) or coal/conveyinggas/nitrogen-containing gas (in the case of oxygen-containing gasinjection) at an angle to the axis of the oxygen lance.

Swirl-Type Injection Element Designs:

The designs presented in this section are based upon the patentedOxynator® (U.S. Pat. No. 5,356,213) concept. It is designed to minimizemixing distance and to prevent high nitrogen or oxygen concentrationsnear the pipe walls.

With respect to the first configuration and as illustrated in FIG. 14,the arrangement of the fuel duct 231 with respect to the conduit 239defined by walls 232A, 232B is a tube within a tube. Nitrogen-containinggas is fed to the central injection element 235 from oxygen lance 236.It is injected with a swirl S2. Oxygen-containing gases fed from conduit239 to the single peripheral injection element 234, which is disposedflush with the inner wall of fuel duct 231. Oxygen-containing gasesinjected from the inner wall of fuel duct 231 with a swirl S1 byinjection element 234. The directions of swirls S1, S2 may the same ordifferent. The flow passage leading to and from the peripheral injectionelement 234 could be aerodynamically (like a venturi) designed to causeminimum disturbance to the flow. In other words, shoulders before andafter the injection element 234 could be used. It should also beunderstood that fuel duct 238 need not extend beyond injection element231A, 231B.

With respect to the second configuration, the conduit 239 may actuallybe a plurality of conduits surrounding the fuel duct 231, any or all ofwhich feeds injection element 234.

As shown in FIG. 15A, another Oxynator®-based design includes fuel duct241 surrounded by a conduit 249 (known by those ordinarily skilled inthe art as a secondary or transition stream zone) defined by walls 242A,242B. Disposed in a central axis of fuel duct 241 is nitrogen-containinggas lance 244 at the end of which is an injection element 244 (basedupon Oxynator®. Disposed along the inner wall of the fuel duct 241 is aplurality of tangentially injecting injection elements 245A, 245B, 245C,245D. In operation, nitrogen-containing gas fed by lance 244 toinjection element 244 is injected into fuel duct 241 with a swirl S3.Oxygen-containing gas fed by conduit 249 to injection elements 245A,245B, 245C, 245D is tangentially injected with respect to fuel duct 241into fuel duct 241 with a swirl S4 that is in the same direction asswirl S3.

As shown in FIG. 15B, another Oxynator®-based design includes fuel duct251 surrounded by a conduit 259 (known by those ordinarily skilled inthe art as a secondary or transition stream zone) defined by walls 252A,252B. Disposed in a central axis of fuel duct 251 is nitrogen-containinggas lance 254 at the end of which is an injection element 254 (basedupon Oxynator®. Disposed along the inner wall of the fuel duct 251 is aplurality of tangentially injecting injection elements 255A, 255B, 255C,255D. In operation, nitrogen-containing gas fed by lance 254 toinjection element 254 is injected into fuel duct 251 with a swirl S5.Oxygen-containing gas fed by conduit 259 to injection elements 255A,255B, 255C, 255D is tangentially injected with respect to fuel duct 251into fuel duct 251 with a swirl S6 whose direction is opposite that ofswirl S5.

All of the Oxynator®-based designs of FIGS. 14, 15A, and 15B may bevaried as follows. As depicted in FIG. 16, injection element Arc 222along the circumferential border of open space 221 between two adjacentvanes 223 has a dimension A1. On the other hand, the circumferentialedge of vane 223 has a dimension A2. The number of vanes 223 and thedimensions A1, and A1 may be varied in order to optimize the mixing andparticle loading. The ratio of dimensions A1, A2 may be chosen tooptimize the injection velocity and thus the penetration of the jet. Asmall ratio A2/A1 is preferred to minimize the disturbance to the solidphase.

Bluff Body Injection Element Designs:

Oxygen-containing gas may be injected at several locations at roughly asingle axial position by several different injection elements.

As shown by FIGS. 17-19, extending from a lance portion 301 is aninjection element comprising a leg member having first and secondportions 302A, 303A and at least one aperture 304A at the end of secondportion 303A. Other injection elements similarly comprise a leg memberhaving first and second portions (302B, 303B; 302C, 303C, 302D, 303D)and at least one aperture 304B, 304C, 304D at the end of the secondportions 303B, 303C, 303D. While not depicted in FIG. 17 for clarity'ssake, an aerodynamic tip 306 is included at the end of lance portion 301just after the junction between lance portion 301 and the first portions302A, 302B, 302C, 302D.

As illustrated by FIG. 19, each injection element has height and lengthdimensions D13, D14. The injection elements inject nitrogen-containinggas or oxygen-containing gas into the fuel duct at an angle P withrespect to an axis of the fuel duct and defined by lines L10, and L11.By strategically placing the injection elements of at various locations,mixing of the oxygen and the coal/conveying gas is enhanced bycontrolling the jet momentum. The cumulative projection area of allthese injection elements perpendicular to the flow area is much smallerthan the flow area of the primary stream. Thus, these injection elementsdo not offer any significant obstruction to the flow of theparticle-laden stream. In this design, the dimensions D13, and D14,injection angle A, and a diameter of each aperture could beindependently adjusted to precisely control the nitrogen-containing gaspenetration or oxygen-containing gas penetration and local mixing.

As depicted in FIGS. 20-21, the first and second portions are replacedwith shapes that are more streamlined. Extending from a lance portion401 are radially spaced fins 402. The side elevation of FIG. 19 depictsa plurality of apertures 403 on surfaces of at least two fins that facein a direction perpendicular to that of the flow of the coal/conveyinggas. However, this type of surface, an opposed surface on the other sideof the fin or a surface of the fin facing downstream could haveapertures 403 to introduce injection gas with precise control over thejet momentum and local penetration of the injection gas.

The lance 402 portion terminates in an aerodynamic body 405 having anaerodynamic tip 406. Each of the fins 402 is aerodynamically streamlinedin shape. The apertures 403 are configured as circular holes, slots,slits, and other shaped openings such as those depicted in FIGS. 3A-3D.

In all the bluff body designs of FIGS. 16-21, the shape of any tip atthe end of the lance has an aerodynamic design with or without one ormore openings. The openings on the tip could be of any design previouslydescribed above.

Axially Injecting Injection Element Designs:

Another type of injection element is configured to injectnitrogen-containing gas or oxygen-containing gas axially into the flowof coal/conveying gas from a surface that faces downstream. This surfacecould have any number of apertures of any shape. Some exemplary shapes701A-F are best shown in FIGS. 24A-F. The number of apertures, size,shape and angle of injection could be adjusted in order to optimizemixing and solid fuel loading.

Baffles arranged near the outlet end can facilitate a uniform mixing ofnitrogen-containing gas and/or oxygen-containing gas (the use of bafflesis an improvement over prior art designs as it accomplishes moreefficient mixing by increasing the turbulence at the outlet end).Various baffles number, shape and size may be utilized. As the velocitycontrol of the jet outgoing from the pipe is a crucial parametergoverning burner aerodynamics, the cross-sectional area of those baffleswill be chosen carefully.

Similar types of axially injecting injection elements have a modifiedcross-section. As gravity has an influence on motion of the particles, avertical elliptical cross-section, for example, will cause fewerdisturbances to the particle trajectories and at the same time couldprovide improved mixing. Modifications of the cross-section of the pipeallow decreasing or increasing the velocity of the axialnitrogen-containing gas or oxygen-containing gas jet. As bestillustrated in FIG. 22A, nitrogen or oxygen lance 503 terminates in ahorizontally oriented elliptical end 502. Similarly, FIG. 22B depicts avertically oriented elliptical end 505.

As depicted in FIG. 23, another axial injecting-type of injectionelement includes member 601 having radially spaced apertures 602A, 602B,602C on a downstream surface. Each of apertures 602A, 602B, 602C isconfigured to inject flows of nitrogen-containing gas oroxygen-containing gas F4, F5, F6 at an angle with respect to an axis ofthe fuel duct.

Preferred processes and apparatus for practicing the present inventionhave been described. It will be understood and readily apparent to theskilled artisan that many changes and modifications may be made to theabove-described embodiments without departing from the spirit and thescope of the invention. The foregoing is illustrative only and thatother embodiments of the method and system may be employed withoutdeparting from the true scope of the invention whose aspects aredescribed in the following claims.

1. A system for burning coal with reduced NOx emissions, comprising: a)a source of a mixture of coal and conveying gas; b) a source ofoxygen-containing gas; c) a source of nitrogen-containing gas; d) aheating device adapted and configured to heat nitrogen from saidnitrogen-containing gas source; e) a combustion chamber; f) a burnerdisposed at a wall of said combustion chamber; a burner operativelyassociated with a combustion chamber; g) a fuel duct in fluidcommunication with said source of a mixture of coal and conveying gas,said fuel duct extending towards said burner; and h) anitrogen-containing gas injection element in fluid communication withsaid heating device and said fuel duct, said first injection elementbeing adapted and configured to inject heated nitrogen-containing gasfrom said heating device into a stream of a mixture of coal andconveying gas and mix therewith inside said fuel duct.
 2. The system ofclaim 1, further comprising an oxygen-containing gas injection elementin fluid communication with said source of oxygen-containing gas andsaid fuel duct, said oxygen-containing gas injection element fluidlycommunicating with said fuel duct downstream of where saidnitrogen-containing gas injection element fluidly communicates with saidfuel duct and upstream of or at said burner, said oxygen-containing gasinjection element being adapted and configured to injectoxygen-containing gas from said oxygen-containing gas source into thestream of mixed heated nitrogen-containing gas, coal, and conveying gas.3. The system of claim 1, wherein said source of oxygen-containing gasand said source of nitrogen comprises an ASU.
 4. The system of claim 1,wherein said conveying gas comprises flue gas from said combustionchamber mixed with oxygen-containing gas from said oxygen source.
 5. Thesystem of claim 1, wherein said heating device is adapted and configuredto directly impart heat to nitrogen-containing gas from saidnitrogen-containing gas source from a flame.
 6. The system of claim 1,wherein said heating device is a heat exchanger adapted and configuredto exchange heat between nitrogen-containing gas from saidnitrogen-containing gas source and heat from combustion of said coal andoxygen in said combustion chamber.
 7. The system of claim 2, whereinsaid conveying gas is flue gas from said combustion chamber mixed withoxygen-containing gas.
 8. The system of claim 2, wherein said heatingdevice is a heat exchanger adapted and configured to exchange heatbetween nitrogen-containing gas from said nitrogen-containing gas sourceand heat from combustion of said coal and oxygen in said combustionchamber.
 9. The system of claim 6, wherein said heating device is a heatexchanger adapted and configured to exchange heat betweennitrogen-containing gas from said nitrogen-containing gas source andheat from combustion of said coal and oxygen in said combustion chamber.10. The system of claim 1, wherein said conveying gas is air.
 11. Thesystem of claim 2, wherein said oxygen-containing gas injection elementfluidly communicates with said fuel duct at said burner.
 12. The systemof claim 1, wherein said nitrogen-containing gas injection elementfluidly communicates with said fuel duct at a peripheral portion of saidfuel duct.
 13. The system of claim 1, wherein said nitrogen-containinggas injection element fluidly communicates with said fuel duct along acentral axis of said fuel duct.
 14. The system of claim 1, wherein saidoxygen element fluidly communicates with said fuel duct at a peripheralportion of said fuel duct.
 15. The system of claim 1, wherein saidoxygen element fluidly communicates with said fuel duct along a centralaxis of said fuel duct.
 16. A method of combusting coal with reduced NOxemissions, comprising the steps of: heating a stream ofnitrogen-containing gas; injecting the heated stream ofnitrogen-containing gas into a stream of coal and conveying gas toproduce a stream of mixed nitrogen-containing gas, coal, and conveyinggas; and combusting the coal from the stream of mixednitrogen-containing gas, coal, and conveying gas with oxygen-containinggas at a burner in the combustion chamber.
 17. The method of 16, furthercomprising the step of injecting oxygen-containing gas into the streamof mixed nitrogen-containing gas, coal, and conveying gas upstream or atthe burner.
 18. The method of claim 16, wherein the nitrogen-containinggas and oxygen are obtained from an ASU.
 19. The method of claim 16,wherein said step of heating a nitrogen-containing gas stream comprisesdirectly imparting heat from a flame to the nitrogen stream.
 20. Themethod of claim 16, wherein said step of heating a nitrogen-containinggas stream comprises indirectly imparting heat from a flame to thenitrogen-containing gas stream via a heat exchanger.
 21. The method ofclaim 16, wherein said step of heating a nitrogen-containing gas streamcomprises indirectly imparting heat from said step of combusting to thenitrogen-containing gas stream via a heat exchanger.
 22. The method ofclaim 16, wherein the stream of nitrogen-containing gas is heated to atemperature in the range of from about 1,000° F. to about 1,800° F. 23.The method of claim 16, wherein injection of the heated stream ofnitrogen-containing gas causes devolatilization of most of a volatilespecies content in the coal.
 24. The method 16, wherein the conveyinggas is air.
 25. The method of claim 16, further comprising the steps of:collecting at least some of any flue gas produced from said step ofcombusting; injection oxygen-containing gas into the collected flue gasto mix therewith; and introducing the oxygen-containing gas oxygen andflue gas to the burner.
 26. The method of claim 25 whereinoxygen-containing gas is injected into the collected flue gas in anamount such that an oxygen concentration in the mixed oxygen-containinggas and flue gas is from about 3% to about 20%.
 27. The method of claim17, wherein said step of heating a nitrogen-containing gas streamcomprises indirectly imparting heat from said step of combusting to thenitrogen-containing gas stream via a heat exchanger.
 28. The method ofclaim 17, wherein the stream of nitrogen-containing gas is heated to atemperature in the range of from about 1,000° F. to about 1,800° F. 29.The method claim 17, wherein the conveying gas is air.
 30. The method ofclaim 17, further comprising the steps of: collecting at least some ofany flue gas produced from said step of combusting; injectionoxygen-containing gas into the collected flue gas to mix therewith; andintroducing the mixed oxygen and flue gas to the burner.
 31. The methodof claim 27, wherein the stream of nitrogen-containing gas is heated toa temperature in the range of from about 1,000° F. to about 1,800° F.32. The method of claim 31, further comprising the steps of: collectingat least some of any flue gas produced from said step of combusting;injection oxygen-containing gas into the collected flue gas to mixtherewith; and introducing the mixed oxygen-containing gas and flue gasto the burner.